Katie,
I think everyone here with any Sierra camping experience will tell you that it can rain, snow or hail any day of the year. I've been hailed on in early August. Unless you are so tough that being wet and cold doesn't bother you, you might want something warm and dry, but it doesn't have to be down, its just that synthetics will weight twice what a down jacket does, but may perform better in above freezing temps. I ski in fleece, camp in down. For any aerobic activity you don't need a lot of insulation, but it should breath well.
Jim